How to rename a text file with user input

I am trying to code my own food selector in python where people can pick if they want a tortilla in their wrap for example and the program will work out the costs.

One of the extra's I am trying to do is that, if the customer does not want to checkout immediately they can save their order to a text file. That all works perfectly but how do I rename this text file to the

user_name

so that only their choices are displayed and so I know who has made these choices?